NAME | SYNOPSIS | DESCRIPTION | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON
VFORK(2) Linux Programmer's Manual VFORK(2)
vfork - create a child process and block parent
#include <sys/types.h>
#include <unistd.h>
pid_t vfork(void);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
vfork(): _BSD_SOURCE || _XOPEN_SOURCE >= 500
(From POSIX.1) The vfork() function has the same effect as fork(2), except
that the behavior is undefined if the process created by vfork() either
modifies any data other than a variable of type pid_t used to store the return
value from vfork(), or returns from the function in which vfork() was called,
or calls any other function before successfully calling _exit(2) or one of the
exec(3) family of functions.
vfork(), just like fork(2), creates a child process of the calling process.
For details and return value and errors, see fork(2).
vfork() is a special case of clone(2). It is used to create new processes
without copying the page tables of the parent process. It may be useful in
performance-sensitive applications where a child will be created which then
immediately issues an execve(2).
vfork() differs from fork(2) in that the parent is suspended until the child
terminates (either normally, by calling _exit(2), or abnormally, after
delivery of a fatal signal), or it makes a call to execve(2). Until that
point, the child shares all memory with its parent, including the stack. The
child must not return from the current function or call exit(3), but may call
_exit(2).
Signal handlers are inherited, but not shared. Signals to the parent arrive
after the child releases the parent's memory (i.e., after the child terminates
or calls execve(2)).
Under Linux, fork(2) is implemented using copy-on-write pages, so the only
penalty incurred by fork(2) is the time and memory required to duplicate the
parent's page tables, and to create a unique task structure for the child.
However, in the bad old days a fork(2) would require making a complete copy of
the caller's data space, often needlessly, since usually immediately
afterwards an exec(3) is done. Thus, for greater efficiency, BSD introduced
the vfork() system call, which did not fully copy the address space of the
parent process, but borrowed the parent's memory and thread of control until a
call to execve(2) or an exit occurred. The parent process was suspended while
the child was using its resources. The use of vfork() was tricky: for
example, not modifying data in the parent process depended on knowing which
variables are held in a register.
4.3BSD, POSIX.1-2001. POSIX.1-2008 removes the specification of vfork(). The
requirements put on vfork() by the standards are weaker than those put on
fork(2), so an implementation where the two are synonymous is compliant. In
particular, the programmer cannot rely on the parent remaining blocked until
the child either terminates or calls execve(2), and cannot rely on any
specific behavior with respect to shared memory.
Fork handlers established using pthread_atfork(3) are not called when a
multithreaded program employing the NPTL threading library calls vfork().
Fork handlers are called in this case in a program using the LinuxThreads
threading library. (See pthreads(7) for a description of Linux threading
libraries.)
The vfork() system call appeared in 3.0BSD. In 4.4BSD it was made synonymous
to fork(2) but NetBSD introduced it again, cf.
http://www.netbsd.org/Documentation/kernel/vfork.html . In Linux, it has been
equivalent to fork(2) until 2.2.0-pre6 or so. Since 2.2.0-pre9 (on i386,
somewhat later on other architectures) it is an independent system call.
Support was added in glibc 2.0.112.
It is rather unfortunate that Linux revived this specter from the past. The
BSD man page states: "This system call will be eliminated when proper system
sharing mechanisms are implemented. Users should not depend on the memory
sharing semantics of vfork() as it will, in that case, be made synonymous to
fork(2)."
Details of the signal handling are obscure and differ between systems. The
BSD man page states: "To avoid a possible deadlock situation, processes that
are children in the middle of a vfork() are never sent SIGTTOU or SIGTTIN
signals; rather, output or ioctls are allowed and input attempts result in an
end-of-file indication."
clone(2), execve(2), fork(2), unshare(2), wait(2)
This page is part of release 3.23 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2009-06-21 VFORK(2)